The present invention generally relates to PGNAA (prompt gamma neutron activation analysis), and more particularly relates to methods and apparatus that apply PGNAA to soil remediation.
With the growing awareness of the contamination of large tracts of land with chemically or radioactively hazardous elements, there is a corresponding international effort to initiate remediation activities to restore affected regions to an environmental status considered acceptable. To this end, soil washing and other methods are being developed. For these methods to be technically efficient and cost effective, it is necessary to accurately identify where the contamination in a field is located. In addition, it is necessary to have, to the greatest extent possible, inline monitoring of remediation process streams, to determine when the treated soil is acceptable for release back to the field. The latter need has been met by a system described in U.S. Pat. No. 5,133,901, Jul. 28, 1992, titled System and Method for On-line Monitoring and Control of Heavy Metal Contamination in Soil Washing Process, which is hereby incorporated by reference into this specification.
Neutron-induced reactions can be divided into two broad categories, threshold reactions and exoergic reactions. For threshold reactions, energy in the form of neutron kinetic energy is required to supply a certain minimum energy to the reaction system before the reaction can proceed. Neutrons with energies below this minimum threshold energy are incapable of inducing the nuclear reaction. For exoergic reactions, the threshold is zero; that is, neutrons with all energies are capable of inducing the reaction. Since neutrons lose energy via nuclear collisions, the minimum energy possible for a neutron is determined by the thermal motion of the atoms in the stopping medium. Neutrons with this minimum average energy are referred to as thermal neutrons and have a mean energy of approximately 0.0252 eV.
FIG. 1 illustrates the process of neutron activation at a nuclear level. A neutron of energy E collides with the nucleus of an atom in the sample and initiates a reaction. For a neutron of thermal energy, the reaction might be absorption of the neutron into the nucleus, creating the next higher mass isotope of that element. If the neutron is more energetic (e.g., with several mega-electronvolts of kinetic energy), other nuclear reactions are possible. These other reactions include inelastic scattering from the nucleus, exciting the atom according to its internal structure of quantum levels, or other reactions ((n,p), (n, alpha), (n,2n), etc.) in which nuclear transmutation to another element occurs. In each of these cases, the residual nucleus is left in a highly excited internal state, and decays to its ground state almost instantaneously (10.sup.-14 seconds or less), emitting a gamma ray of several mega-electronvolts of energy. The energy of this gamma ray is uniquely characteristic of the quantum structure of the residual nucleus, and thus is a signature of the original target nucleus. The number of atoms of each of the elements of interest in a sample can be estimated by detecting and collecting the spectrum of gamma rays emitted by the sample and integrating the appropriate peaks.
The PGNAA process is governed by the following equation: EQU A=N.sigma..phi.B,
where:
A=disintegrations per second producing the desired gamma rays, PA1 N=the number of target nuclei for the reaction, PA1 .sigma.=the reaction cross section (10.sup.-24 cm.sup.2), PA1 .phi.=the flux of neutrons of the required energy (cm.sup.-2 -sec.sup.-1), PA1 B=the branching ratio, i.e., a fraction between 0 and 1 indicating how often this capture produces the gamma ray of interest. PA1 (1) placing a slug of mass M of the target element at a plurality of depths, including: 0 inches; X.sub.1, where X.sub.1 is the deepest depth from which gamma rays of energy E.sub.1 can escape the interrogation volume in sufficient numbers to be detected; and X.sub.2, where X.sub.2 is the deepest depth from which gamma rays of energy E.sub.2 can escape the interrogation volume in sufficient numbers to be detected; and measuring yields Y.sub.E1 (0), Y.sub.E1 (X.sub.1) of gamma rays of energy E.sub.1 at depths of 0 inches and X.sub.1, respectively, and Y.sub.E2 (0), Y.sub.E2 (X.sub.1), Y.sub.E2 (X.sub.2) of gamma rays of energy E.sub.2 at depths of 0 inches, X.sub.1 and X.sub.2, respectively; PA1 (2) defining the following ratios: ##EQU1## performing a field measurement of yields y(E.sub.1), Y(E.sub.2) of gamma rays of energies E.sub.1, E.sub.2, respectively; PA1 Case 1 PA1 if y(E.sub.2)/y(E.sub.1) is greater than R.sub.21 (X.sub.1) and y(E.sub.1) is greater than 0, then the target element is present between 0 and X.sub.1 and between X.sub.1 and X.sub.2 ; PA1 Case 2 PA1 if y(E.sub.2)/y(E.sub.1) is less than R.sub.21 (X.sub.1) and y(E.sub.1) is greater than 0, then the target element is between 0 and X.sub.1 ; PA1 Case 3 PA1 if y(E.sub.2) is greater than 0 and y(E.sub.1) is 0, then the target element is between X.sub.1 and X.sub.2 ; and